The invention relates to a device for subfield coding as defined in the preamble of claim 1.
The invention also relates to a method of subfield coding as defined in the preamble of claim 8.
Such a device for subfield coding is used in large television displays and computer displays, which displays comprise a number of light sources arranged in a matrix. Such a display may comprise a plurality of LEDs or a plasma display panel.
In the known device, the subfield coding is applied to obtain a grey scale by means of pulse width modulation of the light-emitting diodes (LEDs) in the display. The known device comprises means for subdividing every field of an image signal to be displayed into 255 subfields and processing means arranged to program all picture elements of the display device to emit or not to emit light during that subfield. For LED displays, the brightness of the LEDs is dependent on the LED current and the on-period of the LEDs of the respective subfields. Accordingly, a picture element that should produce {fraction (1/256)} of the maximum light output will only emit light during one subfield, and a picture element that should produce {fraction (10/256)} of the maximum light output will emit light during ten subsequent subfields. A disadvantage of this subfield coding is that it provides only a small dynamic range which is not sufficient for typical applications of large television and monitor screens in, for example, open air or in relatively dark control rooms.
Another possibility is to increase the length of each subsequent subfield by a predetermined amount as is applied in, for example, plasma display panels. If, for example, every field of an image signal is subdivided into twelve subfields and the first subfield corresponds to {fraction (1/2048)} of the maximum light output, the second subfield corresponds to {fraction (1/1024)} of the light output and so on, so that a twelve-bit gray scale can be obtained. However, a disadvantage of this subfield coding reduces the maximally obtainable brightness of the display, because the light-emitting elements do not emit radiation for the maximally possible time in a subfield in order to display the maximum brightness.